In recent years, IPv6 (Internet Protocol version 6) which is the next-generation version of IPv4 (Internet Protocol version 4) as a fundamental protocol that supports the current Internet has begun to be examined.
IPv6 is a technology that replaces IPv4, but does not exclude the IPv4 network used so far, and the transition technology from IPv4 to IPv6 has been examined. The transition technology includes tunnel, translate, and dual technologies.
Of these technologies, the dual technology helps smooth transition of devices and applications used on the network by managing IPv4 and IPv6 on a single network.
Upon using IPv4/IPv6-accessible services such as e-mail, Web, and the like in such dual environment, when a server of a communication partner is designated by an IPv4/IPv6 address, a designated communication protocol is used in a “communication protocol selection procedure” that determines one of the IPv4 and IPv6 communication protocols to be used upon making a communication.
On the other hand, when a server of a communication partner is designated by an FQDN (Fully Qualified Domain Name), one of these communication protocols is determined in the following procedure.
That is,    1. A query about an IPv6 address is sent to a DNS.    2. An IPv6 communication is made if acquisition of the IPv6 address has succeeded.    3. A query about an IPv4 address is sent to the DNS if acquisition of the IPv6 address has failed.    4. An IPv4 communication is made if acquisition of the IPv4 address has succeeded.    5. If acquisition of the IPv4 address has failed, the process is terminated as an error since the designated server is absent.
The above communication protocol selection procedure is made either by parsing the operation contents of various existing applications, or with reference to “Basic Socket Interface Extensions for IPv6” (RFC2133, RFC2553) specified by IETF (The Internet Engineering Task Force).
Note that an IPv4/IPv6-accessible server sets both the IPv4 and IPv6 addresses for one network interface, but often registers only one FQDN as a server name.
That is, if an IPv4/IPv6-accessible Web server “www.server.net” exists, both the IPv4 and IPv6 addresses correspond to this server. In the registration contents to the DNS, two records, i.e., “A” record that sets the IPv4 address and “AAAA” record that sets the IPv6 address are often set for “www.server.net”.
Also, most of communication protocols use IPv4 to send a query to a DNS server. For example, as BIND provided by ISC (Internet Software Consortium), BIND4 and BIND8 are used worldwide, but only BIND9 can send a query and response using the IPv6 communication protocol. That is, “AAAA” record can be registered in the DNS, but a query or response cannot be sent using the IPv6 communication protocol.
Also, some OSs install the IPv6 protocol stack. Even such OS or a resolver cannot send any DNS query using the IPv6 communication protocol.
On a network in the dual environment to which clients and server belong, both the IPv4 and IPv6 communications are made on an identical physical line. However, outside that network or outside an ISP which provides that network, IPv4 and IPv6 communications are often made on different lines.
In fact, most of the currently provided IPv6 connection services are implemented tunnel connections.
In the IX (Internet Exchange) of Japan, scarcely any ISPs exchange both IPv4 and IPv6 communication traffics via identical line ports.
Even inside the ISP, an IPv6 backbone is independently built or virtually independent networks are built using the tunnel technology, MPLS, or the like to separately manage IPv4 and IPv6.
In this way, IPv4 and IPv6 communications in the dual environment have equivalent qualities in a single dual network. However, as for communications with an external network, IPv4 and IPv6 communications have quite different bandwidths, convergence levels, and qualities.
Under such circumstance, even when the IPv6 communication is disconnected halfway, a DNS query can be sent to a server as a communication partner using the IPv4 communication protocol. In this case, when that server has “AAAA” record, the IPv6 address can be acquired.
In this manner, when only IPv6 connectivity is lost in a middle network (but IPv4 connectivity is maintained), since the IPv6 address can be acquired, an attempt is made to make a communication using IPv6. However, since a communication with the server cannot be made using IPv6, the IPv4 address is acquired after time-out of several ten seconds, and a communication using IPv4 then starts.
Especially using applications such as a Web browser, e-mail application, and the like that make access frequently, the user may get frustrated since he or she must wait for the end of a time-out time every access.
Upon using IPv6, the dual environment may be recognized as a non-user-friendly environment.